Render Problems

I have been having trouble rendering my scene lately, due to its high poly-count of 15 million faces. After vigorous research and help from the teacher, I managed to solve this problem through a number of changes:

•  editing the fog fluids
• reducing the number of faces on the chains and mountains
• ridding of some unnecessary lights
• altering the position of the ocean 

Now instead of it needing around 7 days to render, it only needs 7 hours and has a poly-count of approximately 400,000.